The polypeptide chains which comprise the subunits of the keratin filaments of normal bovine epidermis have been isolated and characterized. The polypeptides polymerize in vitro into filaments which have the same physical and chemical structure as the in situ keratin filaments. The polymerization in vitro follows an ordered biphasic cooperative mechanism and analysis of the details of this process will provide information on the ultrastructure of the epidermal keratin filament. The polypeptide subunits obtained from abnormal human epidermis are different from those of normal epidermis, implying the existence of abnormalities in the chemistry and structure of the proteins. Further work is in progress to identify such differences. Normal mouse epidermal cells grown in cell culture to study the biosynthesis of the keratin proteins do not synthesize keratin. Rather, the cells synthesize large amounts of a muscle actin-like protein. Interestingly, a variety of human and mouse epidermal tumors also contain large amounts of the actin-like protein. The possibility of using the actin as a biochemical marker in vivo and in cell culture for studies on malignant epidermal tumor is being investigated.